In recent years, the assignee of the present application has developed a new system for steering the front wheels of an agricultural tractor. This system is capable of steering the wheels of the tractor to a degree greater than heretofore possible. This system is called “compound steering” and provides (1) the steering of front wheels with respect to the front axle, and (2) the simultaneous and coordinated steering of the front axle with respect to the chassis.
This system is shown, in U.S. Pat. No. 5,476,276. In the '276 Patent, the two front wheels are mounted on structures that are located on opposite ends of a front axle. These structures are pivotally coupled to the ends of the axle and are steerable with respect to the axle. The structures support the wheels for rotation and are called “wheel hubs” herein.
Hydraulic actuators are fixed to and between the front axle and the front wheels to pivot the front wheels with respect to the axle. Tie rods are coupled to the steering hubs and the chassis. These tie rods force the front axle to steer with respect to the chassis whenever the steering hubs steer with respect to the front axle
The axle itself is T-shaped. It has a rear portion that extends backwards to a pivot joint located one or two feet behind the two front wheels. The entire weight of the vehicle is transferred through this pivot joint up through the T-shaped axle and to the two front wheels disposed at either side of the front axle.
The pivot joint itself is defined by two ball joints shown best in FIG. 2 of the '276 patent. These two ball joints are oriented one above the other to define a generally vertical pivotal axis that passes between the two of them. The front axle is fixed to turn left and right about this axis. These two ball joints constrain the front axle (1) to turn left and right about the vertical axis, and (2) to rotate slightly about a longitudinal and horizontal axis when the lower ball joint is deflected side-to-side slightly at the end of member 35. The system in the '276 patent does not permit any vertical translation of the front axle: whenever the left wheel rises up (with respect to the chassis), the right wheel always drops down (with respect to the chassis) an equal distance, and vice versa.
There are several drawbacks to the design of the '276 patent. First, in order to provide the slight up-and-down pivoting motion together with the side-to-side axle turning motion, the lower ball joint must be supported in an unusual manner. The lower ball joint must simultaneously resist any fore-and-aft movement and permit some slight lateral movement. In order to limit the motion of the lower ball joint, the joint is fixed to an elongated longitudinal member (identified as item 35 in the '276 patent) that extends a considerable distance backwards from the front suspension along the chassis of the vehicle and almost to the rear of the vehicle's engine. This member must be long enough to permit it to bend side-to-side (permitting the front axle to pivot about the longitudinal axis), yet be rigid enough to prevent the lower ball joint from moving fore-and-aft. In effect, item 35 functions as a very stiff spring when pushed in a side-to-side direction and functions as a rigid member when pushed in a fore-and-aft direction.
Another drawback of the suspension shown in the '276 patent is its great weight. The entire load on the front wheels is transferred two feet back to the rear of the T-shaped front axle to the two closely-spaced ball joints. The front axle acts as an elongated, forwardly extending cantilevered beam and therefore must be designed significantly heavier and stronger than a regular axle. This design adds considerably to the weight of the vehicle. It also adds significantly to the unsprung weight of the vehicle, which is always undesirable.
Another drawback of the suspension shown in the '276 patent is the lack of any sprung translational movement of the front axle in a vertical direction. The suspension of the '276 patent does not permit (1) the simultaneous upward movement of the two front wheels with respect to the chassis and (2) the simultaneous downward movement of the two front wheels with respect to the chassis. The front axle cannot translate with respect to the chassis of the tractor.
In the suspension of the '276 patent, both front wheels pivot about a horizontal axis passing through the upper ball joint. This horizontal axis lies in a horizontal plane that is fixed with respect to the chassis of the tractor. As a result, whenever one wheel raises, the other wheel drops. If both front wheels ever drop into a trough or both wheels hit a protrusion such as a rock, a stump, or a curb, the front suspension will not be able to absorb these impacts.
What is needed therefore is a front suspension without a cantilevered front axle. What is also needed is a front suspension having springing and shock absorption. What is also needed is a front suspension that separates the elements used to pivot the front axle about a longitudinal axis and to steer the front axle. It is an object of this invention to provide such a front suspension.